This invention generally relates to the field of semiconductor device manufacturing, and more particularly, to furnaces and the processing of semiconductor devices in furnaces.
Semiconductor device components undergo a number of processing steps that involve heating and possible contamination from residue created during heating steps. Organic residues are generated in furnaces during heating processes involving organic compounds. Heating processes that generate residue contaminants include epoxy resin curing and reflow soldering.
Semiconductor device components, such as flip chips and packages, are conventionally joined together by soldering. In order to efficiently solder in a production environment, solder reflow furnaces are used. In a solder reflow furnace, semiconductor components are placed on a conveyor belt and conveyed through a tunnel-type furnace. The semiconductor components are heated to above the melting point of the solder while in the furnace, permitting the solder to flow and to bond the parts. To provide a good joint the solder is typically heated to about 50xc2x0 C. above its melting point. Solder reflow furnaces are usually operated from about 200xc2x0 C. to about 400xc2x0 C.
Typically, reflow furnaces are heated by either a hot gas or infrared radiation. In general, then, solder reflow furnaces can be classified as radiation-types or hot gas-types. The radiation-type reflow furnace is one in which a number of heater panels are disposed in the upper and lower portions of a tunnel and semiconductor devices are heated by the heat radiated from the heater panels. The furnace is heated to a suitable temperature for soldering by controlling the current supplied to the heater panels. Radiation emitted from the heater panels can either be near or far infrared radiation.
A solder reflow furnace of the hot gas type is one in which hot gas is circulated past a heater providing a constant heating temperature. Typically, nitrogen is the gas of choice in a reflow furnace. However, other inert gases can also be used, as well as hydrogen. Although nitrogen is not a class VIIIA element, as used herein nitrogen will be considered an inert gas. Inert gases and hydrogen are used in both hot gas-type and radiation-type reflow furnaces to limit-the amount of oxygen present in the furnace. Low oxygen levels are desirable to prevent oxidation of the components being soldered.
With reference to FIG. 8, a typical reflow furnace 120 according to the prior art is illustrated. The furnace 120 includes a conveyor belt 126 for passing the semiconductor devices through the furnace. Heating elements 124 heat the furnace to the desired operating temperature. Inert gas enters the furnace through gas inlet 122 and exits through furnace exhaust stack 128.
Before the circuit board enters the furnace 120, solder paste is applied to the areas to be soldered. As well as solder, solder paste includes flux and other additives, such as a solvent. The solder is used to form a metallurgically sound solder joint, which will both hold the various electronic components in place and conduct electrical signals. The flux has a variety of purposes, which include removing oxides from metallization on the circuit board; removing oxides on the molten solder to reduce the surface tension and enhance flow; inhibiting subsequent oxidation of the clean metal surfaces during soldering; and assisting in the transfer of heat to the joint during soldering.
Many problems associated with this process are generated by use of the flux. Depending upon the type of flux paste used, a flux residue can remain after reflow soldering. The residue comprises a carrier, such as rosin or resin that is not evaporated, acid or salt deposits, and the removed oxides. If not removed, this residue can be detrimental to the long-term reliability of an electronic package. The resin can also absorb water and become an ionic conductor, which could result in electrical shorting and corrosion. Additionally, the residual flux can, over a period of time, corrode the soldered components and cause electrical open circuits.
Several different types of fluxes are commonly used in the semiconductor manufacturing industry, including cleanable and no-clean fluxes. Cleanable fluxes include resin-type fluxes, such as Alpha 102-1500 manufactured by Alpha Metals, and water soluble fluxes. When a flux is used that leaves corrosive and/or hygroscopic residues, post-soldering cleaning using chlorinated fluorocarbons (CFCs), organic solvents, semi-aqueous solutions, or water is required. For this type of process, in addition to volatile organic compound emissions from the soldering process, the cleaning process results in emission of CFCs and wastewater. These emissions detrimentally add to environmental pollution and production costs.
Recently no-clean fluxes have begun to be used in the reflow soldering industry. Instead of the residue remaining on the circuit board after reflow welding, these no-clean fluxes are designed to undergo chemical decomposition at a given temperature, also known as pyrolyzation, during which the residue is volatilized into the furnace atmosphere. Because this flux leaves little or no residue, the need to clean the circuit board after reflow welding is negated.
A problem associated with all types of fluxes is that the volatilized residue vapor in the furnace atmosphere tends to condense very quickly onto cool surfaces within the reflow furnace.
Volatilized flux is carried out through the furnace exhaust stack by the gas circulating in the furnace. In a high volume production environment, some of the volatilized flux condenses as a residue on the inside walls of the furnace and the inside walls of the furnace exhaust stack. As the build-up of the flux residue increases, it starts to drip from the furnace walls and exhaust stack-walls and falls onto the production parts, contaminating the parts. Flux residue contamination of production parts will cause rejection of the parts. Flux residue contamination of production parts leads to a lower yield of production parts, and therefore, higher manufacturing costs.
The term semiconductor devices as used herein are not be limited to the specifically disclosed embodiments. Semiconductor devices as used herein includes a wide variety of electronic devices including flip chips, flip chip/package assemblies, transistors, capacitors, microprocessors, random access memories, etc. In general, semiconductor devices refer to any electrical device comprising semiconductors.
There exists a need in the semiconductor device processing art to control and/or eliminate the problem of residue contamination of production parts in a furnace.
This and other needs are met by the embodiments of the present invention, which provide a method of preventing residue contamination of semiconductor devices during furnace processing. Semiconductor devices are conveyed through a furnace comprising an exhaust stack. Gas is flowed through the furnace and exits through the exhaust stack. Residue build-up is monitored using a device attached to the exhaust stack. A signal is generated by the device when the amount of residue build-up reaches a predetermined amount.
The earlier stated needs are also met by another embodiment of the instant invention that provides a furnace. The furnace comprises heating elements that heat the furnace, a conveyor that conveys semiconductor devices through the furnace, and an exhaust stack for venting gases from the furnace. A device for monitoring residue build-up in the furnace is attached to the exhaust stack.
The residue build-up monitoring device monitors the amount of residue build-up on the walls of the exhaust stack. Through empirical determinations, the amount of residue build-up on the walls of the exhaust stack is correlated to the amount of residue build-up on the inner furnace walls.
The residue build-up monitoring device uses light beams to monitor the amount of residue build-up. The transmittance of a light beam across the exhaust stack is measured when the walls are free of residue build-up. This baseline information is stored in the device. The device continuously monitors transmittance of the light beam across the exhaust stack. When the transmittance drops to a predetermined level, indicating that residue build-up has reached the point where removal of the residue build-up is required to prevent contamination of production parts, the device transmits a signal to a display notifying technicians that cleaning of the furnace and exhaust stack is required. At this point, technicians can clean the furnace and exhaust stack before any contamination of production parts occurs.
Some advantages of the instant invention include improved reliability of semiconductor devices, reduced production costs due to fewer rejected parts, and an increased production yield due to fewer rejected parts. The furnace of the present invention provides cleaner semiconductor devices with higher yields. The instant invention provides a more efficient manufacturing process in which fewer parts will require reworking. In addition, productivity enhancement is also accomplished by accurate determination of the furnace cleaning schedules. Furnace cleaning is only performed when needed.
The foregoing and other features, aspects, and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.